1. Field of the Invention
The present invention relates to a digital display measuring apparatus, and particularly to such a measuring apparatus including an electrostatic type encoder.
2. Prior Art
There is well known a measuring system for digitally displaying measurements. The digital display measuring system can eliminate any personal error upon reading and quickly read measurements. Therefore, such a type of measuring systems have currently been utilized in various different fields of industry.
In general, the digital display measuring system comprises a measuring element movable on the system body and adapted to contact an object to be measured, an encoder for detecting the movement of the measuring element and converting it into an electrical signal pulse, a counter circuit for counting the electrical signal pulse from the encoder, and a digital display for digitally indicating the counted value from the counter circuit. Thus, the measurement can instantly be read.
The known encoder used in this type of measuring systems includes photoelectric type encoders, contact type encoders, electrostatic type encoders and others.
The photoelectric type encoder comprises a rotary disc including slits formed therein and spaced away from one another at regular intervals, and light emitting and receiving elements between which the disc is disposed and which form an optical path through the slits on the rotary disc. The rotary disc is adapted to rotate depending on the displacement of the measuring element. As the disc is rotated, it intermittently blocks the optical path so that the displacment of the measuring element will be detected.
In the photoelectric type encoder, however, the light emitting element consumes an increased amount of power. If the system is supplied with its power from a battery, the battery must frequently be replaced by a new battery. Also, if a battery having its larger capacity is used in the system, the entire system will be increased in size. When it is desired to improve the accuracy of measurement, a great number of slits must be formed in the rotary disc several microns apart. This is difficult to manufacture and yet tends to provide errors due to changes in clearance during operation.
The contact encoder utilizes a slit, a brush and others on detecting of the displacement of the measuring element. The slit and brush are violently worn. Also, the measurement signal frequently contains a noise.
On the contrary, the electrostatic type encoder provides a power consumption less than that of the photoelectric type encoder and also has no problem with respect to the brush, slit and others which are violently worn and provide noises as in the contact type encoder. Therefore, the electrostatic type encoders have broadly been utilized in various digital display measuring systems.
In the conventional digital display measurement systems, the electrostatic type encoder comprises at least a pair of electrode plates positioned faced to each other to form a capacitor. These electrode plates are adapted to move relative to each other depending on the displacement of the measuring element. This relative displacement between the electrode plates is converted into the change of electrostatic capacity in the capacitor which is in turn detected electrically.
For example, a plurality of electrode plates having one polarlity are arranged side-by-side on a main scale at regular intervals. The main scale is disposed parallel to and spaced away from an index scale on which a single electrode plate having the other polarity is positioned. One of the main and index scales is adapted to move depending on the displacement of the measuring element. On the relative displacement of the scales, therefore the capacitor defined by the electrodes will be changed in capacity. This is electrically detected as a displacement in the measuring element.
In the conventional arrangement of the electrostatic type encoder, the capacitor defined by the electrodes is utilized to define a voltage dividing circuit which is adapted to detect a ratio of divided voltages varying depending on the change of electrostatic capacity in the capacitor. This detected ratio is used to know the displacement of the measuring element. Where the distance between the electrode plates defining the capacitor for any other reason or where the voltage applied to the voltage dividing circuit is changed, the output of the dividing circuit will not accurately indicate the displacement of the measuring element.